The gastrointestinal tract of mammals is densely colonized by hundreds of microbial species that coexist symbiotically with their hosts. The microbes, collectively referred to as the microbiota, contribute to numerous aspects of host health, including nutrient metabolism, homeostasis of intestinal tissues, development of innate and adaptive immune responses, and more generally, defense against intestinal infection. Bacteria antagonize intestinal pathogens directly, through contact-dependent and soluble factor-mediated inhibition, as well as indirectly by calibrating and inducing host immune responses, but the contributions of individual bacteria to colonization resistance against specific pathogens are not well understood.
Enterococci are a genus of gram-positive, round-shaped bacteria that commonly live in the gut, although they can cause infection anywhere in the body. The genus has a high amount of intrinsic resistance to some classes of antibiotics, but is generally sensitive to vancomycin. However, particularly virulent strains that are resistant even to vancomycin are an emerging and are a growing problem, particularly in institutional settings. Such vancomycin-resistant strains are referred to as VRE. The two main VRE species are vancomycin-resistant Enterococcus faecium (E. faecium) and vancomycin-resistant Enterococcus faecalis (E. faecalis). VRE can exist in the body, typically in the gastrointestinal tract, without causing a disease or other harmful effects. However, VRE can sometimes cause local disease in the gastrointestinal tract and they can invade sites outside the gastrointestinal tract and cause disease, for example, in the bloodstream, abdomen, or urinary tract. VRE in the bloodstream can be particularly problematic because, once in the bloodstream, VRE can cause sepsis, meningitis, pneumonia, or endocarditis.